(1) Field of the Invention
This invention relates generally to audio volume controls and relates more specifically to an IC audio volume control comprising an operational amplifier and one or more R/2R ladder networks.
(2) Description of the Prior Art
IC audio volume controls are typically implemented by creating a potentiometer, which is used for the input and feedback resistors round an operational amplifier as shown in FIG. 2 prior art. The arrangement is favored over those which attenuate the input signal by means of a potentiometer to VCM, as shown in FIG. 1 prior art, as this results in the volume section switches having to carry current, so they have to be large in order to achieve audio linearity requirements. This arrangement also guarantees monatomic gain steps, an important consideration in an audio volume control.
For noise performance in the −100 dBV region resistances of 40 kΩ are required. To achieve an attenuation of 60 dB, resistor ratios of 1000 to 1 are required; this results in a requirement for low value resistances with accurate matching to high value resistances. To achieve this, a large number of unit resistors are typically used in various series and parallel combinations.
It is a challenge for the designers of audio systems to achieve circuits for audio volume controls requiring minimum silicon area, accurate volume gain settings, and a gain range that can be extended to very high levels of attenuation with very little area overhead.
There are known patents or patent publications dealing with circuits for audio volume controls:
U.S. Patent (U.S. Pat. No. 6,448,856 to Noro et al) proposes an electronic volume circuit, which can be driven by a single power source and can therefore be formed by an LSI that can be fabricated in a simple manner using an oxide film and a junction process for a single power source. A first amplifying circuit attenuates the amplitude of a bipolar input signal and converts the attenuated input signal to a unipolar signal, and a variable resistor device controls the degree of attenuation of the first amplifying circuit based on an externally supplied signal.
U.S. Patent (U.S. Pat. No. 6,807,406 to Razavi et al.) discloses a receiver system with a variable gain mixer circuit that is advantageous over current architectures used in wireless communication systems. The use of a variable gain mixer circuit simplifies the receiver architecture resulting in the elimination of additional circuit blocks and a reduction in complexity and cost. Moreover, one embodiment of the present invention includes a mixer circuit comprising a mixer core, a bias circuit coupled to the mixer core for providing a bias current, and a variable impedance network. The mixer core receives input signals and generates output currents that are coupled to the variable impedance network. Each of the output currents is selectively coupled to a voltage output node through variable impedance. Variable gain is established by varying the impedance between the output currents of the mixer core and the voltage output node.
U.S. Patent Publication (US 2008/0212802 to Shih et al.) describes an audio processing system comprising an audio processor and an audio amplifier. The audio processor receives a data signal to generate a processed signal, and comprises at least one gain control circuit and at least one operational amplifier. The gain control circuit generates a gain signal according to a volume control signal, a reference signal, and a feedback signal. The operational amplifier couples to the gain control circuit and amplifies the data signal by the gain signal to generate a processed signal. The audio amplifier couples to the audio processor to receive and amplify the processed signal, wherein an amplified signal is generated.
Furthermore the following patents are known:
U.S. Patent (U.S. Pat. No. 7,298,855 to Tsuji et al) proposes a volume circuit containing resistive ladder circuits, from which a desired fractional voltage output (Vs) is extracted and supplied to an amplifier to provide an output voltage (Vout). The resistive ladder circuits comprise multiple lines of series resistances, wherein each line contains a resistance portion ‘nR’ (where ‘n’ denotes a division index, and ‘R’ denotes an element resistance) that is connected in parallel with a next line, so that an overall resistance of following lines is (n−1)×n times larger than the element resistance.
U.S. Patent (U.S. Pat. No. 7,162,029,855 to Soman et al) discloses a gain or input volume controller and method including a modified R2R ladder network having a number of R2R branches, switches coupled respectively to the R2R branches, and a switch controller for respectively controlling the switches to control and provide an overall gain value for a signal. The switch controller further includes a mapper for mapping a gain control signal to the switches wherein the gain control signal respectively activates or deactivates the switches.
U.S. Patent (U.S. Pat. No. 6,693,491 to Delano) teaches a control circuit for controlling a level of an audio signal and transmitting the signal to an amplifier. The control circuit is based on an R-2R resistor network having a first plurality of resistor nodes and a parallel resistor network having a second plurality of resistor nodes. Each of the resistors in the parallel network has a value equal to one-half of the value of the preceding resistor.
U.S. Patent (U.S. Pat. No. 6,147,558 to Sculley) describes two banks of differently connected resistors, which are connected as an input to an op-amp feedback circuit.
U.S. Patent (U.S. Pat. No. 6,127,893 to Llewellyn et al.) discloses a control circuit for controlling a level of an audio signal and transmitting the signal to an amplifier. The control circuit is based on an R-2R resistor network having a plurality of resistor nodes. A plurality of switches alternately connects each of the plurality of resistor nodes to one of a plurality of low impedance nodes and a low impedance input node associated with the amplifier. Switch control circuitry selectively controls the plurality of switches to transmit the audio signal to the low impedance input node.